1. Field of the Invention
This invention relates to an improved method for curing epoxy resins and the resin product thus produced.
2. Description of Prior Art
U.S. Pat. No. 3,467,393 to Legler (1969) teaches the use of a polyoxyalkylenepolyamine for curing a polyglycidyl ether of a polyhydric phenol. Homan's U.S. Pat. No. 2,783,214 (1957) discloses the curing of an epichlorohydrin-bisphenol resin-forming material at ambient temperature with a specific mixture of amines, i.e., a major amount of triethylene tetramine and a minor amount of triethanolamine, in the presence of a highly reactive substance which will polymerize spontaneously in the presence of the stated amines.
In several applications, accelerated curing of epoxy resins at ambient temperatures is essential. A typical situation is the use of an epoxy resin as an adhesive in a flammable environment. Many known prior art processes for preparing low temperature cured epoxy resins are disadvantageous inasmuch as the resultant epoxy resins have inferior physical properties, e.g., brittleness, loss in resistance to water absorption, strength, and the impairment of electrical properties. On the other hand, epoxy resins of the polyglycidyl ether of a polyhydric phenol type cured with a polyoxyalkylenepolyamine exhibit superior physical properties. However, curing with polyoxyalkylenepolyamines at ambient temperatures is often too slow for use in many applications.
There are a few prior art processes known for accelerating the cure of epoxy resins cured with a polyoxyalkylenepolyamine. For example, Lee, Henry and Neville, Kris, Handbook of Epoxy Resins, McGraw-Hill Book Co., N.Y., 1967, p. 7-15 describes the use of N-(2 aminoethyl) piperazine as an epoxy curing agent accelerator. U.S. Pat. No. 3,639,928 to Bentley et al. discloses a process for accelerating the curing of epoxy resins cured with a polyoxyalkylenepolyamine by incorporating therein an accelerator mixture of N-(3-aminopropyl) piperazine and salicylic acid. U.S. Pat. No. 3,875,072 describes an acceleration combination of piperazine and an alkanolamine.
However, known prior art processes and accelerators still, at times, may be too slow for many applications of epoxy resins requiring cures at ambient temperatures. On the other hand, it has been found that epoxy resins can be cured at ambient temperature with a polyoxyalkylenepolyamine in very short periods of time by using the improvement of an accelerator combination of the instant invention.